The typical electrical power distribution underground network is in the form of a grid having multiple network vaults, commonly referred to as network centers, connected together on the secondary, or customer, side of the network transformers to provide service for multiple customer loads. The system's primary side is connected to source of electrical power which is typically from electric utility substation and includes multiple network transformers in a vault fed from separate primary feeders connected in parallel on the secondary side. The substations are connected via primary network feeders on the primary side of the network to provide service to multiple network centers. The network transformers are connected with together via secondary cables also called secondary mains. The secondary mains, the customer load, are typically connected by means of inline current limiting fuses in series with service cables. The grid network centers provide a high level of redundancy. When one or more transformers in the network center become disabled or de-energized, the customer's load is maintained by balance network transformers in the vault center that remain in service. The secondary voltage in the grid network is designated as either 120/208V or 277/480V, where the first value in each expression indicates the voltage relative to (neutral) ground, and the second value in each of the aforementioned expressions indicates the voltage relation of the 3 phase's to each other. It is important that the phase angle of each of the phase voltages in each of the three cables be equal, or as close as to equal as possible, to provide maximum energy transmission efficiency with the three phases more or less equally sharing the load, as well as for safety considerations. Differences in phase angle between the operating voltages of the network feeder cables can result in damage to the network. In extreme cases even result in the destruction of the forced paralleling of out of phase feeders system components. Out of phase angle conditions also pose a very dangerous situation for workers who maintain the network. Great care is taken by electric utilities to maintain the phase angle between network primary feeder voltages at a minimum yet this condition does persist and this invention proposes a safe operating solution.
The aforementioned network protectors are designed to trip open when backfeeding current an abnormal operating condition. When large differences in voltage or phase angle are detected in paralleled network feeders, the network protector isolates the backfed transformer from the other transformers to which it is connected. The secondary network protector normally automatically isolates a transformer exhibiting abnormal operating conditions (backfeeding) from the secondary network system in response to predetermined electrical conditions controlled by a master relay. Network protectors are subject to malfunction or a lock-up condition during operation which permits the load current to flow in the opposite direction from the flow direction in normal operation, so as to direct the current flow from the low voltage secondary side to the high voltage primary side resulting in a highly dangerous condition in the networked group of high voltage network feeder lines. The present invention is directed to quickly and safely isolating a malfunctioning network protector from the network to allow the network protector to be cleared without an interruption in electric service or endangering those who maintain and repair the network.
A locked backfeeding network protector must be opened to solve both load flow problems and clear the device for sale repair.
Present approaches to isolating a malfunctioning network protector include the following alternatives:
(1) Interrupting all power served by the single network center resulting in a shutdown of electric service to all customers served from that vault. This presents a difficult situation for customers, particularly in large buildings having elevator systems with elevators stranded between floors.
(2) Another approach involves opening substation circuit breakers servicing the primary network feeders involved and interrupting all service to connected customers and then manually opening the defective secondary protector and associated network transformer primary switch.
Another method for achieving the above is to leave the primary connections as is and proceeding as follows:
1. Manually set the substation supply voltage regulators to minimize the backfed current on the defective unit through voltage control.
2. Parallel the isolating secondary fuse links with a breaker or switch to transfer the load current for isolation by unbolting the transformer side fuse links, one phase at a time.
3. With fuse links open, the defective protector can be manually opened.
4. With the defective secondary protector open, the interlock protection is now disabled and the associated network transformer primary switch can be opened, interrupting only magnetizing current and grounded for safe repair.
To prevent network feeder collapse, the time frame for using this procedure to clear a faulty network protector is limited to a very light load period and is not undertaken at any other time. In addition, the three network transformer protector clearing approaches discussed above have been the subject of OSHA complaints relating to the unsafe conditions to which workers are exposed in isolating a faulty network protector.
Single phase load dropping can also be precarious with possible arcing associated with load interruption on three phase ever-changing load levels. While less dangerous than the 3 phase isolation of a network protector, the single phase approach is also dangerous because of the lengthy time lag associated with completion of de-energizing of all the phases. The time required to control load flow with primary voltage regulation in a substation having as many as four primary network feeder sources can result in excessive heating of the various electric components, including the load carrying cables, resulting in damage to, or destruction of, exposed network components. These conditions are particularly dangerous in close proximity to a live malfunctioning network protector.
The present invention addresses the aforementioned limitations of the prior art by providing a quick, safe and automatic approach to resolving the problem of a defective network protector in an electrical power distribution network.